In general, this invention relates to a process which increases the concentration of nitric acid in an aqueous solution by forming additional nitric acid therein. As hereinafter disclosed, what is proposed involves, basically, a method of jointly conducting reactions having functions which include: oxidation of nitric oxide to form higher oxides of nitrogen; oxidation of reduction products from the foregoing oxidation; and formation of nitric acid, in aqueous solution, from the aforesaid higher oxides of nitrogen and water. Also disclosed is how this method is closely integrated with an extractive distillation process using sulfuric acid.
It is assumed that an exhaust gas stream containing water vapor and nitrogen oxides derived by means of ammonia combustion according to known technology would be the most convenient source, together with a source of oxygen, of feedstream materials either for start-up of a batch production version of a plant incorporating the invention, or for sustaining a continuous mode of production. However, it is feasible to apply the invention in conjunction with alternative sources of oxygen, water, and nitrogen oxides.
The invention more especially relates to a process wherein a negligible portion of total nitric oxide consumed is oxidized according to the known "homogeneous, non-catalytic, gas-phase reaction between nitric oxide and additional oxygen to produce nitrogen dioxide" which is discussed at length by Thomas H. Chilton in THE MANUFACTURE OF NITRIC ACID BY THE OXIDATION OF AMMONIA. Unlike other gas phase reactions this one, instead of going faster as temperature is raised, proceeds as a termolecular reaction with a rate constant which diminishes at increasing temperature, thus requiring resort to means to elevate gas pressure--an application of Le Chatelier's Principle--as in the `Du Pont Pressure Process`, in order to oxidize nitric oxide without deceleration of the rate of reaction.
However, an alternative avenue to producing needed higher oxides of nitrogen has been explored by a number of below-cited workers in the art, who represent a divergent branch of nitric acid manufacture wherein the focus is not on means for homogeneous gas phase oxidation of NO. Instead, their apparatus arrangements and processing conditions are directed to a concentrating technique based essentially on interrelating two reactions of the following natures: (a) formation of higher oxides of nitrogen, accompanied by liberation of bound water, by means of reducing nitric acid originating in the condensate of ammonia burner exhaust, using nitric oxide as the reducing agent which is itself oxidized; and (b) formation of nitric acid, together with co-product nitric oxide, by means of water dissolution of higher oxides of nitrogen. Such an approach would be infeasible to conduct in a batch mode of production, because the mass balance situation would entail getting nowhere, but in several issued patents the approach is purported to have utility in continuous production modes.
Among the practitioners of this diverging branch of art, surmisably there would be opinions that certain chemical events regarded hereinafter as undesirable are better understood as irremediable `givens` necessitating separation of reaction locales, hence the various fluid recirculation systems of the prior art. The undesirable events are not irremediable, it turns out.
One such chemical event is gas effluence; another is solution dilution. It would be expected that co-produced nitric oxide from formation of nitric acid in an aqueous solution would `gas off`, and this could even be viewed as helpful if the plan anyway is to carry the gas onward to another reactor site in a plant, eg. by entraining it into an air-stripping stream. But that is not the plan herein adopted, and NO effluence is viewed as a key problem requiring resolution.
Another key problem also concerns gas effluence, though now effluence of the higher oxides of nitrogen, but in this case accompanied by dilution of reaction solution due to chemical reduction of nitric acid and liberation of water. When the plan anyway is to conduct an absorption-type reaction at a reactor site elsewhere in a plant, this dilution too could be understood as not a problem, or even helpful (at a particular stage). Neither is that the plan herein adopted, however. The problem requires resolution.
These problems of dilution and effluence are associated respectively--although with an overlap regarding effluence--with the two reactions to which attention has already been drawn by way of characterizing their natures (a) and (b) supra, and which next are displayed for didactic purposes, reproducing an evidently unbalanced type of molecular equation notation exactly as presented in a below-cited patent (by M. J. Kalous): EQU NO+2HNO.sub.3 -&gt;3NO.sub.2 +H.sub.2 O [I] EQU 3NO.sub.2 +H.sub.2 O-&gt;2HNO.sub.3 +NO [II]
To the clearly shown reversal of reaction direction, may be plausibly attributed an apparent intention among prior art workers employing both reactions, that they should either be conducted at spatially separated locations in different solution bodies, or else at least that there should be provision for separate zones in a current of fluids in which one or the other reaction substantially predominates, though both may occur to some extent in the same flow. Evidence of such intentions is easily detected in apparatus arrangements.
In another below-cited patent (A. Christiansen), notation for describing the same reaction (I) does not similarly fail (as above) to indicate the number of water molecules, reading instead: "NO+2HNO.sub.3 -&gt;3NO.sub.2 +3H.sub.2 O"; yet in another patent (L. M. Rodrigo et al), notation for the same reaction reads "2 NO.sub.3 H+NO+Heat-&gt;3NO.sub.2 +H.sub.2 O", again omitting the information that the number of molecules of water liberated is the same as the number of molecules of nitrogen dioxide evolved. It seems that when the context makes it clear that aqueous solutions of nitric acid are involved, some though not all describers of the reaction (I) which reduces nitric acid and produces water along with higher oxides of nitrogen omit from notation both the textbook recommended `(aq)` after `2HNO.sub.3 `, and the number of water molecules directly involved. Surmisably, such notation practice may reflect acceptance of dilution as a given and/or de-emphasis on the magnitude thereof. Seeing such equation notation practices in the art, one should also recall that HNO.sub.3 in aqueous solution is completely ionized.
Discussions of reaction (II) in the pertinent prior art frequently give an impression that it can be indifferently regarded whether this reaction involves water with or else without nitrate ion present. But then apparatus arrangements manifest a strong preference for water containing nitrate ion.
More closely looked at in order to advance the art, such reactions known in the prior art are between free radicals and nitrate ions in water, on the one hand; and between free radicals and water molecules in the presence of nitrate ions, on the other hand--where reactant NO and higher oxides of nitrogen, respectively, comprise the free radicals referred to here.
Sometimes in the literature (eg. the patent of Oberste-Berghaus et al, cited below), there is no equation given for reaction [I] and where it is employed is only signaled by mention of `washing out`. And, in much of the literature, the term `absorption` signals where reaction [III] is employed. Such conventions of usage may perhaps tend to hinder giving close attention not just to free radical-ion reactions but to macroscopic events accepted as inevitable, such as gas effluence and dilution accompanying reactions.
A third problem area of pertinence requires separate mention, namely: how best to integrate one or another kind of distillation process with a system forming nitric acid in aqueous solution. The options are clear.
In America, the preference is extractive distillation using a strong dehydrating agent, commonly sulfuric acid, whereby a water-nitric acid mixture of any concentration can be separated completely, leaving no azeotrope. Some objections have been raised to this approach. In U.S. Pat. No. 3,542,510 (Newman et al), for example, it is stated with regard to extractive distillation: "Such procedures . . . require large amounts of steam or other heat sources . . . In addition, the use of a dehydrating agent introduces an extraneous chemical agent into the nitric acid system."
The drawback of thermal demands requires attention, of course, but is mitigated even with application of ordinary skill. The thought-provoking issue of an "extraneous chemical agent" seems to voice laudable preference for tightly integrated schemes, and it is appreciated that extractive distillation apparatus and procedures can in fact be merely aggregated with almost any kind of nitric acid manufacturing system, without affecting its principles of operation as a real combination would. It does not follow, however, either that a truly integrated scheme is infeasible, or that sulfuric acid sourced from an extractive distillation unit cannot be used elsewhere than in that unit, non-extraneously.
The other option: ordinary distillation of water-nitric acid mixtures, does appear easier to closely integrate into nitric acid systems per se. Chiefly outside the United States, there are many plants which distill hyperazeotropic nitric acid in a manner yielding a dual output of practically pure nitric acid `overhead` (for sale) and azeotrope `bottoms` (for recycling back into an absorption process.) Such `Direct Strong Nitric` acid concentrating schemes in some respects share common ground with the present invention, although the common ground has nothing to do with which mode of distillation is employed. When there is avoidance of homogeneous gas phase oxidation of nitric oxide, that constitutes the common ground.
Attention is drawn to the following U.S. Pat. Nos.: 1,901,816 (Luscher); 2,098,953 (Christensen); 3,399,965 (Kalous); 3,716,625 (Oberste-Berghaus et al); and, 4,064,221 (Rodrigo et al). By itself it would not say much: that these prior art inventions disclose resort to reaction [II], in view that virtually the whole larger field of art does. However, these also rely on reaction [II] for oxidation of nitric oxide, and of special relevance is the way a mutually enhancing relation between the two reactions is established: characteristically by means of apparatus-related provisions comprising recirculation piping for physical removal of diluted acid solution from a locale where reaction (I) occurs, or at least is the predominant reaction, taking the diluted result elsewhere for participation of its free water content in acid formation according to reaction [II]. Characteristically also is provision to remove nitric oxide from some reaction [II] locale where solution concentration of nitric acid is increased--taking evolved NO back to the locale of reaction [I].
With reference to the Figure in U.S. Pat. No. 3,399,965 (Kalous), at the right side of the Figure, inside "absorption tower 12" there is a vertical bypass "conduit 37" which together with recycling "line 41" provides physical means whereby the inventor obtains mutually enhancing effects of reactions (I) and (II). Perhaps easily overlooked but of special relevance is an unmentioned consequence of the staggered disposition of "conduit 37" and "line 41", respective nearest adjacent outlets and inlets of which are spaced apart. In spaces therebetween, in lower regions of both "absorption zone 29" and "oxidation zone 32", substantial amounts of all reactants for both reactions (I) and (II) are unavoidably present together, manifesting a degree of departure from other inventors' more stringent measures to ensure isolation of the two reactions. Also, this patent shows reactions (I) and (II) both conducted in a single reactor column designated an "absorption" tower though it really is an oxidation tower as well, ie. processes both of oxidation of nitric oxide at the cost of diluting some acid, and of the usual absorption type which increases concentration but evolves NO, are conducted in the same tower 12. This ingenius azeotrope-producing tower is clearly the more important one of the two towers depicted, for its inventor suggests that a system embodying his invention can consist of just this one tower, merely by increasing its size and admitting water to the top. (cf. Col. 7, lines 21-25)
Now referring to the Figure of U.S. Pat. No. 4,064,221 (Rodrigo et al), the conveyance to and fro of reaction products from separate locales manifests a higher degree of intention (than in the invention by M. J. Kalous) to spatially isolate locales respectively for nitric oxide oxidation by means of nitric acid reduction (most intensely conducted in 7), and for formation of new acid (most effective in tower 9, but also conducted in tower 10).
The exploited reactions designated (I) and (II) by M. J. Kalous are designated respectively (IV) and (III) in this citation, which is also pertinent because it illustrates a sub-trend within art employing these reactions: combination with a distillation system--but opting for an easier to integrate ordinary mode. Such objections as are raised in the patent of Newman et al thus do not apply. Insofar as is known from searching, no art within the trend of exploiting what the patent by Rodrigo et al points to as "inverse" reactions discloses integration with extractive distillation.
The invention of Rodrigo et al apparently exemplifies total omission of means for homogeneous gas phase oxidation of nitric oxide, especially if an assertion at Col. 3, lines 30-32, that cooling of reagent gases in heat exchangers 2, 3, and 4 is "irrelevant for the purposes of the patent", is accepted at face value. Somewhat curiously, although it is stated that advantages also accrue from "NO not being handled in the form of a gas" (Col. 2, line 66), the effectiveness of compressor 8 which handles NO certainly depends on it being in the form of a gas. What was really meant may be something like: `NO formation not being handled by resort to gas phase oxidation.` Notwithstanding the noted curious contrary-to-fact remark (likely an inadvertent translation error), the handling of gas as it is in fact handled would clearly not be impeded by evolution of gas at the top of oxidation system 7, the case being quite the contrary.